Alveolar sacs (magnified)

Sacculi alveolares

RespiratoryOrgansWhole bodyRight side

View in 3D

What it is

Grape-like clusters of alveoli at the ends of the alveolar ducts, where the airway ends in thin-walled air sacs.

Function

Gas exchange: oxygen diffuses from alveolar air into capillary blood and carbon dioxide diffuses out across a barrier only ~0.5 µm thick.

Size

Each alveolus is about 0.2–0.3 mm across; the lungs hold roughly 300–500 million, a surface of ~70 m².

Clinical note

Type II cells make surfactant that stops alveoli collapsing; premature babies lacking it develop respiratory distress syndrome.